A static VAR generator using thyristor-switched capacitors is described in U.S. Pat. No. 4,234,843 issued Nov. 18, 1980 and entitled "Static VAR Generator With Discrete Capacitive Current Levels" and a control scheme for this is detailed in U.S. Pat. No. 4,307,331, issued Dec. 22, 1981 and entitled "Hybrid Switched-Capacitor Controlled-Inductor Static VAR Generator and Control Apparatus". As known, in this type of static VAR generator a number, in general n, of capacitor banks is employed across an ac electrical network as illustrated in FIG. 1. Each capacitor bank includes the series combination of a capacitor, Cn, a bidirectional thyristor-switch, SWn, and a surge current limiting inductor, Ln. The capacitor bank with the series inductor can be switched to the ac supply network without any transient if the capacitor is charged to a specific voltage, which is the amplitude of the steady-state ac voltage across the capacitor bank in conduction, and the switching takes place exactly at the peak of the ac supply voltage. This ideal switching condition is satisfied in practice only if the capacitor bank is switched back to the network shortly (within a few cyles) after it was disconnected, that is, before the capacitor residual charge could change appreciably. At disconnection under ideal switching conditions, the capacitor is charged to the peak of the ac network voltage and discharges from this value slowly via the internal discharge resistance of the capacitor. Eventually the capacitor will reach a completely discharged condition.
Switching of the capacitor bank to the ac electrical network when it is fully or partially discharged will generate an oscillatory transient. This transient can be minimized, but not eliminated, by initiating the capacitor switching at the time instant when the capacitor voltage and the ac voltage are approximately equal, that is, when the voltage across the thyristor switch is approximately zero. If the LC circuit formed from the capacitor and the surge current limiting inductor has a high quality factor (or Q factor) then the oscillatory disturbance due to the non-ideal switching conditions would exist for a long time. An oscillatory transient in the capacitor voltage Vc due to switching in a high Q thyristor-switched capacitor circuit with a discharged capacitor is illustrated in FIG. 2. The current I and ac network voltage V waveforms are also shown. The long oscillatory disturbance in the capacitor voltage Vc, which can have a large amplitude under particular switching conditions, can cause a severe distortion in the network voltage, control problems for the VAR generator, and increased stresses for the power components of the VAR generator and of the ac network. To avoid these problems, the surge current limiting reactor is normally shunted by a damping resistor, R, as shown in FIG. 3A, to reduce the Q factor of the thyristor-switched capacitor circuit. In this way the oscillatory transient in the voltage Vc decays relatively quickly, as illustrated in FIG. 3B for the previous example of switching-in a discharged capacitor bank. However, the damping resistor, R, which has to be rated for high transient voltage and relatively high power dissipation, increases the cost and reduces the efficiency of the static VAR generator appreciably. Accordingly, it would be advantageous to be able to eliminate the need for the damping resistor.It is one object of the present invention to provide a switching technique for a thyristorswitched capacitor that would achieve trasient damping without the use of a damping resistor resulting in reduced cost and increased efficiency for the static VAR generator.